This invention relates generally to switching amplifiers and, in particular, to a switching amplifier which uses a plurality of independently referenced output stages in a bridged configuration for greater accuracy at reduced cost.
Switching amplifiers have historically relied upon the principle of modulating connection time within a fixed control period between a unipolar or bipolar power supply reference and the load. The voltage or current available to the load is essentially that of the connected power supply multiplied by the connection duty cycle (ratio of ON time to total time). An output voltage, current, or power is thus controlled in a proportional fashion by a modulating input, which is updated as frequently as once per control period. The control period typically uses ranges from {fraction (1/20)}th to xc2xd the period of the highest frequency of interest.
The outputs of switching amplifiers are filtered before application to the load. This reduces heterodyne products between the modulating source and the control period (aliasing). Use of shorter control periods allows use of lower-Q output filters, whereas longer control periods mandate sharp filter slopes. Filter costs therefore encourage the use of shorter control periods.
In the past decade, the dynamic range of sources for amplification has increased dramatically. A typical digital audio source now has a dynamic range of 96 dB and a bandwidth of 20 kHz. In order to accurately amplify such a signal, a typical switching amplifier output stage then requires maximum timing resolution of roughly one part in 65,000 within a 40 kHz control frequency, a resolution substantially less than one nanosecond.
FIG. 1 shows a typical prior-art bridge switching amplifier output stage using a single reference for both output drivers. FIG. 2 shows the timing requirements of the amplifier output stage, wherein waveforms 202-205 correspond to the (active-high) drives applied to the electrically controlled switches 102-105 in FIG. 1. Power supply 101 is connected to switches 102 and 104 in order to control voltage to the load 108 through either filter 106 or 107. The return path of power supply 101 is connected to switches 103 and 105 in order to complete the path.
Control of switches 102 through 105 is effectuated by control circuit 109. Filters 106 and 107 serve to remove switching components from the output. Switches 102 and 103 are activated exclusively with a specific duty cycle, while switch 105 is activated to provide one polarity to the load 108, via filters 106 and 107. Alternatively, switches 104 and 105 are activated exclusively with a specific duty cycle, while switch 103 is activated to provide an opposite polarity to the load 108 through filters 106 and 107.
In the event that the output stage of FIG. 1 is operated at a typical rate of 100 kHz, the time resolution to accurately amplify a signal with 96 dB dynamic range (one part in 65536) is seen to be the reciprocal of 100 kHz divided by 65536, or 153 picoseconds. This is beyond the capability of common-available silicon semiconductors. A different approach is indicated in order to accurately amplify high-bandwidth, high-accuracy signals at reasonable cost.
This invention resides in a switching amplifier incorporating a plurality of independent output stages in a bridged configuration. Broadly, each output stage presents the product of an independent duty cycle and two or more static or dynamic reference voltages, currents, or powers to a single terminal of a common output load. The configuration achieves higher accuracy at lower cost than conventional designs by utilizing multiple references to produce an output voltage, current, or power with relaxed timing requirements.
In the preferred embodiment, first and second voltage references are used, with the voltage of the first reference being higher than the voltage of the second reference. The first voltage reference is higher than the second voltage reference by a factor that is a power of two. A plurality of electrically controlled switches interconnect the references to the load, and waveform generator controls the switches in a manner whereby the first voltage is applied for a coarse control of power to the load and the second voltage is applied for a fine control of power to the load. The waveform generator preferably uses pulse-code modulation (PCM), though the invention is not limited in this regard, and is applicable to any modulation scheme or waveform suitable to sequence the electrically controlled switches.
The load is preferably filtered on either side, and returns paths for the power supplies are connected through separate switches connected to the load as filtered. In an embodiment utilizing two references, a system according to the invention preferably includes a first pair of electrically controlled switches used to connect one reference to each side of the load in series, a second pair of electrically controlled switches used to connect the other reference to each side of the load in series, and a third pair of electrically controlled switches, one on either side of the load for retum-path purposes. The waveform generator in this case includes sufficient outputs to control each switch as appropriate.